The present invention relates to a semiconductor device including a high voltage semiconductor power element such as a diode or an IGBT.
One of high voltage semiconductor elements used in high voltage semiconductor devices for performing power control is a high voltage diode. FIG. 1 is a sectional view showing a conventional high voltage diode.
In FIG. 1, reference numeral 81 denotes a first n-type cathode layer (semiconductor substrate) having a high resistance. A first p-type anode layer 82 is selectively formed in the front surface of the n-type cathode layer 81. A second heavily doped p-type anode layer 83 is selectively formed in the surface of the first p-type anode layer 82.
A lightly doped p-type resurf layer 84 with a field relaxation structure (junction termination structure) is formed around the p-type anode layer in the front surface of the n-type cathode layer 81 in contact with the p-type anode layer. A heavily doped n-type channel stopper layer 85 is formed outside the p-type resurf layer 84 in the front surface of the n-type cathode layer 81 so as to be spaced apart from the p-type resurf layer 84 by a predetermined distance.
A high-resistance film 86 is formed in the region extending from an edge of the second p-type anode layer 83 to the first p-type anode layer 82, the p-type resurf layer 84, the n-type cathode layer 81, and the n-type channel stopper layer 85. Instead of the high-resistance film 86, an insulating film may be formed.
A second n-type cathode layer 87 doped more heavily than the n-type cathode layer 81 is formed on the back surface of the first n-type cathode layer 81 having a high resistance. A cathode electrode 88 is formed on the n-type cathode layer 87. An anode electrode 89 is formed on the second p-type anode layer 83, whereas an electrode 90 is formed on the n-type channel stopper layer 85. Reference numeral 91 denotes an insulating film.
A conventional high voltage diode of this type however suffers the following problems. That is, to increase the withstanding voltage, the n-type cathode layer 81 must be made thick. As the n-type cathode layer 81 becomes thicker, the forward voltage drop and the reverse recovery loss increase, resulting in poor element characteristics. In the worst case, the device may be destroyed.
In recent years, needs for smaller-size, higher-performance devices for switching circuits such as inverter circuits and chopper circuits are becoming stronger.
FIG. 2 shows the main circuit arrangement of an inverter using a conventional IGBT (Insulated Gate Bipolar Transistor). Since the inverter circuit contains an inductance component in its load, like motor control, energy stored in the inductance of the load must be discharged upon selective turning off of switching elements (IGBTs in this case) Tr1 to Tr4. To reflux the electrical energy, freewheeling diodes D1 to D4 are connected anti-parallel to the IGBTs.
In this conventional semiconductor device, a junction termination region having a predetermined area or larger must be set within a semiconductor chip in order to obtain a withstanding voltage equal to or higher than the power supply voltage in each pair of IGBT and freewheeling diode. For this reason, the chip area is difficult to reduce, failing to increase the current density. To make a module including the semiconductor device (e.g. IGBT), a separate element as a freewheeling diode is externally connected to the IGBT. That is, an IGBT chip and a freewheeling diode chip are mounted on a single board, and electrodes on the respective chips and external electrodes are connected via wires. In this arrangement, a high-speed operation cannot be attained owing to the inductances of the connecting wires.
For the IGBT as well, demands for small loss arise. FIG. 3 is a sectional view showing the arrangement of an IGBT of this type. In this IGBT, a p-type drain layer 102 is formed on one surface of a high-resistance n-type base layer (semiconductor substrate) 101. A p-type base layer 104 is selectively formed in the other surface of the n-type base layer 101, and an n-type source layer 105 is formed in the p-type base layer 104. A gate electrode 107 is formed on the p-type base layer 104 between the n-type base layer 101 and the n-type source layer 105 via a gate insulating film 106. The gate electrode 107, the gate insulating film 106, the p-type base layer 104, the n-type base layer 101, and the n-type source layer 105 constitute an electron injection MOSFET having a channel region CH1. A drain electrode 108 is formed on the p-type drain layer 102, and a source electrode 109 is formed on the n-type source layer 105 and the p-type base layer 104.
The operation of this semiconductor device will be described below. While positive and negative voltages are respectively applied to the drain and source electrodes 108 and 109, if a positive voltage with respect to the source is applied to the gate electrode 107, the surface of the p-type base layer 104 opposite to the gate electrode 107 is inverted to be of the n type. Electrons e are injected from the n-type source layer 105 into the n-type base layer 101 via the inverted layer to reach the p-type drain layer 102. Along with this, holes h are injected from the p-type drain layer 102 into the n-type base layer 101. In this manner, both the electrons e and the holes h are injected into the n-type base layer 101 to cause conductivity modulation, which allows to reduce the ON voltage.
In a turn-off operation, a negative voltage with respect to the source is applied to the gate electrode 107. Then, the inverted layer formed immediately below the gate electrode 107 disappears to stop the injection of electrons. Some of the holes h in the n-type base layer 101 are discharged to the source electrode 109 via the p-type base layer 104, and the remaining holes h recombine with the electrons e and disappear. As a result, the semiconductor device is turned off.
In the conventional IGBT, however, the electrons e and the holes h must travel beyond a potential barrier formed by the p-n junction between the n-type base layer 101 and the p-type drain layer 102 in the conductive state. That is, as shown in the current-voltage graph of FIG. 4, the ON resistance increases by a built-in voltage of about 0.7V in proportion to a voltage drop caused by the p-n junction. In the conventional IGBT, therefore, the ON resistance in the conductive state cannot be satisfactorily reduced.